Directional couplers are general purpose tools used in radio frequency (“RF”), microwave, and millimeter wave signal routing for isolating, separating or combining signals. They find wide application in RF, microwave, millimeter wave, and optical frequency networks and systems. They perform a variety of functions including, for example, splitting and combining power in mixers, power monitoring and sampling power from sources for level control and source leveling, isolating signal sources, separating incident and reflected signals in network analyzers, allowing for swept transmission and reflection measurements, and dividing power among a number of loads.
In general, directional couplers are devices that have two transmission lines that are physically positioned close together. These transmission lines may be, for example, coaxial transmission lines, waveguide transmission lines, optical transmission lines, and stripline and microstrip transmission lines. In operation, the electromagnetic field of one transmission line is utilized to couple energy into the second transmission line. Based on then design, directional couplers couple a predetermined amount of power input on the first transmission line to the second transmission line which is typically referred to as the coupled transmission line.
For planar-transmission-line structures, directional couplers are usually constructed utilizing microstrip or stripline transmission lines which may be constructed on a printed circuit board (“PCB”). In FIG. 1, a prospective view of an example of an implementation of a known directional coupler 100 constructed on a PCB utilizing microstrip transmission lines 102 and 104 is shown. The PCB includes a dielectric substrate 106, which has a thickness h 107, and ground plane 108. The first transmission line 102 and second transmission line 104 are spaced closely together with a spacing s 110 (generally known as the “gap”) apart. Both the first transmission line 102 and second transmission line 104 have a width w 112 and a thickness (not shown) above the substrate 106. In this example, the directional coupler 100 is a four port passive device having an input port 114, through port 116, coupled port 118, and isolated port 120. It is appreciated by those skilled in the art that the directional coupler 100 may be housed in a shielded box and coaxial-transmission-line connectors may be bonded to each port 114, 116, 118, and 120 of the microstrip transmissions lines 102 and 104. The dielectric substrate 106 may be, for example, fused silica or SiO2.
Typically the directional coupler 100 is constructed in a backward configuration in that a signal input into the input port 114 propagates between the input port 114 to the through port 116 (also known as an output port) via the fast transmission line 102. When the signal propagates along the first transmission line 102 it creates an electromagnetic field which couples energy onto the second transmission line 104. Some of the electromagnetic field crosses the second transmission line 104 and the electric field from the first transmission line 102 includes an equal and opposite charge on the second transmission line 104 giving rise to an electric field that is reversed in direction from that of the first transmission line 102. Since the there is a reversal in the electric field, there is also an accompanied reversal in the direction of propagation along the second transmission line 104. As such, while the signal input into the input port 114 propagates along the first transmission line 102 from the input port 114 to the through port 116, the coupled signal induced on the second transmission line 104 propagates in the opposite direction (i.e., in the direction from the isolated port 120 to the coupled port 118). For this reason, the directional coupler 100 is usually known as a backward coupler because it utilizes the backward wave coupling principle, which means that its coupling direction of propagation is opposite the propagation direction of the main signal. It is appreciated by those skilled in the art that the direction coupler 100 may be analyzed utilizing techniques involving analyzing the even-symmetry and odd-symmetry modes of operation of the transmission lines 102 and 104 along a plane of symmetry defined along a line of symmetry 122. Based on this approach, the odd and even modes of the resulting combined signal waveform on the pair of coupled transmission lines 102 and 104 travel at the same velocity but, due to the different characteristic impedances of the transmissions lines 102 and 104, cancel at the isolated port 120 and combine constructively at the coupled port 118 and through port 116.
Turning to FIG. 2A, a front-side view of the direction coupler 100 is shown. In FIG. 2A, the electric field 200 and magnetic fields 202 for the even-symmetry mode on the coupled microstrip transmission lines 102 and 104 are shown. Similarly, in FIG. 2B, a front-side view of the direction coupler 100 is also shown. In FIG. 2B, the electric field 204 and magnetic fields 206 for the odd-symmetry mode on the coupled microstrip transmission lines 102 and 104 are shown.
In this example, the direction coupler 100 is designed to pass most of the energy input into the input port 114 of the first transmission line 102 to the through port 116. A portion of the energy (which is not passed to the through port 116) will be coupled to the second transmission line 104 with most of that coupled energy being passed to the coupled port 118; however, some of the coupled energy will also be passed to the isolated port 120. In practice, the direction coupler 100 will be designed to have a pre-determined amount of energy passed to both the through port 116 and coupled port 118 while at the same time minimizing the amount of energy passed to the isolated port 120. The design parameters and techniques for designing the direction coupler 100 are well-known by those skilled in the art and include, for example, varying the transmission line 102 and 104 widths w 112, gap spacing s 110, length l 124 of the coupling sections, shapes, bends, thickness, and materials of the transmission lines 102 and 104, properties of the substrate 106, etc.
In general, the directional coupler 100 is characterized by its coupler factor, isolation, and directivity. Its coupling factor is defined as the ratio of power obtained at the coupled port 118 and the power input into the input port 114. In mathematical form the coupling factor is described as
      C    =                  -        10            ⁢              log        ⁡                  (                                    Power              CoupledPort                                      Power              InputPort                                )                    ⁢      dB        ,where dB stands for decibels. The coupling factor represents a primary property of a directional coupler 100. It is a negative quantity (even though in practice the minus sign is frequently dropped) and it cannot exceed 0 dB for a passive device. Additionally, the coupling factor is not constant and varies with frequency.
Similarly, the isolation of the directional coupler 100 is defined by the ratio of power obtained at the isolated port 120 and the power input into the input port 114. In mathematical form the isolation of the direction coupler 100 is described as
  I  =            -      10        ⁢          log      ⁡              (                              Power            IsolatedPort                                Power            InputPort                          )              ⁢          dB      .      The isolation should be as high as possible to reduce the amount of power being transmitted to the isolation port (i.e., isolated port 120).
Directivity is directly related to isolation and is the ratio of power obtained at the isolated port 120 and the power obtained at the coupled port 118. Again, in mathematical form the directivity of the directional coupler 100 is described as
  D  =            -      10        ⁢          log      ⁡              (                              Power            IsolatedPort                                Power            CoupledPort                          )              ⁢          dB      .      
As a result of this, the directivity may also be described as the ratio of the isolation and coupling factor of the directional coupler 100. In mathematical form this would be written as
  D  =            I      C        ⁢          dB      .      
In general, directivity has been widely used as a figure of merit to quantify the quality and usefulness of a directional coupler. The directivity should be as high as possible for a properly designed directional coupler. It is appreciated by those skilled in the art that for a tightly coupled coupler (such as, for example, a 3 dB directional coupler), a high directivity is not difficult to achieve. Unfortunately, this is not true of a loosely coupled directional coupler such as, for example, a 13 dB directional coupler.
As an example, a 3 dB directional coupler only needs to have an isolation of 18 dB to achieve 15 dB directivity. However, for a 13 dB directional coupler to achieve the same 15 dB directivity it will need to have at least 28 dB isolation for the entire operating frequency of the directional coupler. This high isolation requite lent for a broadband loosely coupled high directivity coupler becomes extremely challenging in the design of a conventional backward directional coupler.
It is well known that to design a high directivity directional coupler, it is necessary to satisfy the following relationshipZ0eZ0o=Z02,
where Z0e and Z0o are the even and odd mode impedances of the couple-line structure and Z0 is the characteristic impedance of the directional coupler 100. In addition to the Z0eZ0o=Z02 relationship, the even and odd impedances also need to follow a certain design profile as the coupling sections 126 and 128 moving away from the input port 114 toward the through port 116. This usually results in an asymmetric form factor with a very tight gap spacing 130 at one end and wide gap spacing 132 at the other end. The gap spacing between the two coupled lines 126 and 128 controls the amount of coupling between each small section of the coupler along the signal propagation direction. A wrong spacing between the two coupled lines 126 and 128 translates directly to the wrong even and odd mode impedances and results in a different characteristic impedance Z0. This can cause undesired reflection at each port and can degrade the isolation of the directional coupler 100. Depending on the dielectric material used in the substrate 106, this tight gap 130 at the input side may be extremely small and hard to build especially for a low dielectric constant material.
Since the directivity is the ratio between the isolation and coupling factor, in order to produce a high directivity it is important that the isolation be as high as possible. It is well known that in a directional coupler in the backward configuration, a good isolation is achieved by the cancellation of the two different propagation modes (even and odd) at the isolation port (i.e., isolated port 120). This cancellation relies on a matched propagation velocity between the two modes. If the two modes do not have the same propagation velocity, then when the two modes propagate to the isolation port they will not be perfectly cancelled. This results in a degraded isolation and lowers the directivity of the directional coupler.
To achieve a matched propagation velocity, known approaches have included utilizing stripline and air dielectric slab line structures to preserve the transverse electromagnet (“TEM”) mode and minimize the difference of the two propagation velocities. With a pure TEM mode supported by these two structures, the backward directional couplers can have the same propagation velocities between the even and odd modes, therefore, a high directivity coupler may be achieved. However, stripline and slab line couplers may not be a perfect solution for a pure planar hardware implementation in a RF, microwave, or millimeter wave circuit.
Many times a microstrip structure is a preferred way to implement in hardware. However, by the nature of microstrip structure, it does not support a pure TEM mode. Without a pure TEM mode, the two propagation modes will not be cancelled at the isolation port of the directional coupler. Therefore, a high directivity backward directional coupler is difficult to achieved and built using standard microstrip technology.
In order to solve this problem a few known approaches have been developed to match or minimize the difference of the propagation velocities in the backward directional coupler utilizing microstrip construction. These approaches include utilizing a wiggled coupling structure to slow down the odd mode, using coplanar waveguide (“CPW”) to lower the difference of the two propagation velocities, and a combination of both. Unfortunately, these approaches are complex and do not properly solve the problem. As an example, the approach of utilizing a wiggled coupling structure requires the utilization of extra wiggled saw teeth to add extra length for the fast moving odd-mode. This adds extra design challenges to the already difficult problem. The CPW line solution only lowers the difference of the two propagation velocities and does not fully solve the issue. Attempts at combining both of these approaches have included implementing the wiggled sections to compensate the propagation velocity and also utilizing a suspended CPW line to further lower the difference of the two propagation velocities. Unfortunately, this approach further increases the complexity of the overall circuit. It also requires extra bonding wires along the coupled structure to equalize the CPW's two separated ground plans and prevent higher propagation modes.
As a result, there is a need for an improved loosely coupled directional coupler that provides high directivity.